1. Field of the Invention
The present invention relates to Layer 2 switches, and more specifically, to a Layer 2 switch for copying a multicast packet, in multicast packet transfer used for content information distribution such as broadcasting.
2. Description of the Related Art
In the initial stage of the spread of the Internet among the general public, Internet access was obtained in most cases by a direct dial-up connection to an access point of an Internet service provider (ISP) and authentication. A commonly used protocol is Point-to-Point Protocol (PPP), which supports an authentication function and a compression function. PPP is a Layer-2-class protocol for making a one-to-one connection between a user terminal and an ISP access point. In this age of broadband, a connection by an access carrier network using Internet Protocol (IP) has become the mainstream of connection between a user terminal and an ISP, instead of a connection by a telephone network. Layer-1 protocols such as Ethernet (registered trademark) and Asynchronous Transfer Mode (ATM) are used in the access carrier network. For these n-to-n protocols, PPP, which was originally developed for one-to-one authentication, cannot be used directly. PPP on Ethernet (registered trademark (PPPoE) and PPP on ATM (PPPoA) have been developed for those applications.
As broadband networks have advanced, multicast technologies for broadcasting and other content distribution have also been receiving attention. The conventional multicast technologies utilizing IP include an IP multicast method and a PPP multicast method. Efficient information distribution can be conducted by using an IP-multicast router or an IP-multicast access server for implementing a broadcast service on an IP network built for communication. As technologies for multicast communication for distributing data to a plurality of specific destinations, the Internet Engineering Task Force (IETF) has standardized TCP/IP details in Internet Group Membership Protocol (IGMP: RFC 1112, chapter 4, chapter 7, appendix 1 (non-patent document 1), RFC 2236, chapter 2, chapter 3, chapter 6, chapter 7 (non-patent document 2)) for IPv4, and Multicast Listener Discovery (MLD: RFC 2710, chapter 3, chapter 4, chapter 5, chapter 6 (non-patent document 3) for IPv6.
IGMP and MLD are used between a user terminal and a packet transfer apparatus (such as a gateway and a router). The protocols control a group of user terminals (a multicast group) made for receiving multicast distribution, which distributes identical data to a plurality of user terminals efficiently. IGMP and MLD are used when a user terminal makes a request to join a multicast group (multicast data distribution request) or when a user terminal makes a request to leave a multicast group (request to stop multicast data distribution). When a content distribution server distributes information in packets, by a request sent from a user terminal included in a multicast group, a content distribution service is performed as follows: A low-level packet transfer apparatus (such as a gateway or a router) receives the packet, makes as many copies of the packet as the number of user terminals included in the requesting multicast group, and transfers the copies to the user terminals. Copying a multicast packet by an apparatus closest to the user terminal is ideal for minimizing a network load in multicasting. Japanese Unexamined Patent Application Publication No. 2003-69639 (patent document 1) describes an x Digital Subscriber Line (xDSL) apparatus including a cache server and a method of storing received content data in the cache server of the xDSL apparatus on behalf of the clients and distributing data copied by the xDSL apparatus to each client. This method requires a large-capacity cache that can hold a moving image and other data. Japanese Unexamined Patent Application Publication Nos. 2003-32287 (patent document 2), 2003-348148 (patent document 3), and 2003-152796 (patent document 4), and others are also known to the public.
In a general access carrier network, a user terminal is generally connected to a broadband access server (BAS) for the purpose of authentication and further connected through a router of an ISP network to a content distribution server. In some other cases, a user terminal is connected to a Layer 2 Tunneling Protocol (L2TP) access concentrator (LAC) and further connected through an L2TP network server (LNS) to an ISP network. The BAS or LNS (hereafter referred to as a packet transfer apparatus) keeps track of user terminals connected to it by IGMP or MLD and performs multicast data distribution by making as many copies of each multicast packet received from the content distribution server as the number of user terminals and sending the copies at the same time.
If the packet transfer apparatus used in the network has a small number of ports, the user terminals may be connected by a Layer 2 switch, and the user terminals and the packet transfer apparatus may be connected by PPPoE, VLAN, and others. IGMP or MLD is used to keep track of the user terminals in this network as well, but IGMP or MLD gives layer information of IP or a higher level. A Layer 2 switch does not support packet processing of IP or a higher level and cannot keep track of the user terminals connected to it. Accordingly, a packet transfer apparatus supporting Layer 3 or above must be used to manage the user terminals and distributes packets to the user terminals connected to it. However, since a point-to-point (P2P) connection is used between the user terminal and the packet transfer apparatus, unicast communication performed for distribution to the user terminals would use up an n-fold bandwidth. So, a sufficient bandwidth must be reserved between the Layer 2 switch and BAS, resulting in a high cost.